Photoinduced Charge Transfer Dynamics in Mixed-Dimensional van der Waals Heterostructures
by Dr. Christopher E. Petoukhoff (Post-doctoral Researcher, Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology, Japan)
Tuesday, June 11, 2019 from to (Asia/Kolkata)
In recent years, there has been growing interest in van der Waals (vdW) materials – layered materials with weak inter-layer bonding that can be exfoliated down to monolayer thickness, giving the material two-dimensional (2D) electronic properties. Semiconducting vdW materials in particular have attracted tremendous attention due to their quantum confinement effects, giving rise to thickness-dependent optoelectronic properties as the thickness approaches the monolayer limit. Due to the lack of dangling bonds at their surfaces, semiconducting vdW materials are ideal candidates to form p-n heterojunctions by stacking individual layers together, enabling them to form ideal, defect-free interfaces without the need for energy-intensive growth processes, such as molecular beam epitaxy. Ultrathin devices have already been fabricated from vdW heterostructures, including sub-nanometer-thick light-emitting diodes, solar cells, and transistors. However, because the typical exfoliation process results in monolayers that are only few microns in area, there are limitations to employing vdW heterostructures in large-area devices with high optoelectronic quality. This challenge can be overcome by stacking 2D vdW materials with materials of different dimensionalities, such as organic semiconductors, which have either zero- (0D) or one- (1D) dimensional electronic properties. These so-called, “Mixed-dimensional vdW heterostructures” thus enable the fabrication of ultra-thin devices while benefitting from scalable processing technologies. Organic semiconductors, whether conjugated polymers (1D) or small molecules (0D), also lack dangling bonds at their surfaces and interact via vdW forces, allowing them to form p-n heterojunctions with 2D semiconductors. Organic/2D heterojunctions have demonstrated remarkable device characteristics when employed as active p-n heterojunctions in optoelectronic devices (e.g., photovoltaics, photodetectors and field-effect transistors), such as large on-off ratios, anti-ambipolar transfer characteristics, and record values of photovoltaic figures of merit normalized to the device active layer thickness. Despite this, these heterojunctions still display low internal quantum efficiencies, suggesting non-ideal charge transport through the device. Understanding the charge transfer dynamics across organic/2D semiconductor interfaces at fundamental time scales is an important part of overcoming these limitations. In this seminar, we will discuss our recent studies investigating the photoinduced charge carrier dynamics in organic/2D heterojunctions comprised of solution-processed organic semiconductors and large-area monolayer MoS2. First, using photoluminescence and femtosecond transient absorption spectroscopy, we compare the efficiencies of charge transfer for three different conjugated polymer/MoS2 heterojunctions: P3HT, PCDTBT, and PTB7. We show that electron transfer occurs from MoS2 to P3HT in under 9 ps, and from MoS2 to PCDTBT or PTB7 in under 120 fs. We demonstrate that P3HT/MoS2 heterojunction is the most efficient because the transferred charges have an order-of-magnitude increase in their lifetimes, giving rise to enhanced photoluminescence. In the second part of the talk, we show that by employing plasmonic metasurfaces, we enhance the charge generation by 6-fold, and increase the total active layer absorption bandwidth by 90 nm. We demonstrate that organic/MoS2 heterojunctions can serve as hybrid solar cells, and their efficiencies can be improved using plasmonic metasurfaces.